When pianist, pedagogue and Arnold Schoenberg's former secretary Leonard Stein formed Piano Spheres in Los Angeles in 1994, he was providing exposure to four of the best and most imaginative students he had mentored.

Gloria Cheng, Vicki Ray, Mark Robson and Susan Svrcek are now among the best, busiest and least dispensable pianists in Los Angeles, to say nothing of the most dedicated.

Nine years after Stein’s death, Piano Spheres still thrives and on Saturday celebrates the series’ upcoming 20th anniversary by providing a rare opportunity to hear all four pianists on the same stage.

The program, titled “Dear Leonard: a Musical Tribute,” is in the Piano Spheres tradition of being full of interest and relevance, with pieces ranging from early Schoenberg to John Harbison’s 2009 “Leonard Stein Anagrams.”

One rarity will be Webern’s arrangement of the Prelude to Schoenberg’s “Gurrelieder” for two pianos, eight hands. That would be worth the price of admission alone, were there a price of admission. But the UCLA Music Library has selected to use its Hugo Davise Fund for Contemporary Music to make this concert on campus in Schoenberg Hall free to the public.